Triggering Alarms Tutorial for Android: Getting Started

Rescheduling an Alarm When the Device Restarts

By default, the system cancels all alarms when a device shuts down. To prevent this, improve your app by rescheduling alarms when the user reboots the device.

Open AndroidManifest.xml, and replace TODO (22) with:

<uses-permission android:name="android.permission.RECEIVE_BOOT_COMPLETED" />

When the system finishes booting, it broadcasts ACTION_BOOT_COMPLETED. This allows your app to receive that event and react to it. Keep in mind that this only works if the user has already launched the app at least once.

Next, open RescheduleAlarmsBroadcastReceiver.kt, and replace TODO (23) with:

class RescheduleAlarmsBroadcastReceiver : BroadcastReceiver() {

  override fun onReceive(context: Context, intent: Intent) {
    val action = intent.action
    if (action != null) {
      if (action == Intent.ACTION_BOOT_COMPLETED) {
        (context.applicationContext as StuddyApplication).exactAlarms.rescheduleAlarm()
        (context.applicationContext as StuddyApplication).inexactAlarms.rescheduleAlarms()
      }
    }
  }
}

This adds a whole new BroadcastReceiver, which will react on ACTION_BOOT_COMPLETED events.

Finally, go back to AndroidManifest.xml. Replace TODO (24) with:

<receiver
    android:name="com.yourcompany.android.studdy.alarm.RescheduleAlarmsBroadcastReceiver"
    android:exported="true">
  <intent-filter>
    <action android:name="android.intent.action.BOOT_COMPLETED" />
  </intent-filter>
</receiver>

With this code, you define and register the BroadcastReceiver in the AndroidManifest.xml. It’ll reschedule your alarms after the system finishes booting.

Now, if you schedule any alarm, and restart the device, the alarms are still scheduled.

Exploring Best Practices

Now that you’ve completed the app, you’ll go over some of the best practices when working with alarms.

Considering System Resource Consumption

Triggering exact alarms is expensive. When the system triggers exact alarms, the device consumes a lot of resources, such as battery charge, especially if it’s in a power-saving mode. Furthermore, the system can’t easily batch these requests to use resources more efficiently. When creating an exact alarm, always consider using an inexact alarm instead.

Understanding Acceptable Use Cases For an Exact Alarm

Use exact alarms only when your app has user-facing functionalities that require a specific action to be invoked at a precise time. For example:

• An alarm clock app or a timer app.
• A calendar app that shows notifications for upcoming events.

Using Alarms For Network Requests

If you plan to trigger network requests depending on a repeating alarm, add some randomness to when they’re called. If every instance of your app on multiple devices pings the server at the same time, your server might not be able to handle the load.

Alarm Time Precision

When specifying the time for alarms, don’t make it any more precise than necessary. Always think about how precise your app needs to be when working on a specific use case.

Avoid Basing Your Alarm on Clock Time

Repeating alarms that are based on a precise trigger time don’t scale well. When possible, use ELAPSED_REALTIME instead.

Where to Go From Here?

Download the final project using the Download Materials button at the top or bottom of this tutorial.

Congrats! You learned how to use the AlarmManager API, and how to implement exact and inexact alarms — great job!

If you want to learn more about background processing on Android, make sure you check out the Android Background Processing video course that also covers scheduling work with AlarmManager. Check Scheduling Tasks With Android WorkManager as well to learn more about WorkManager.

If you have any questions or comments, please join the discussion below! :]